1. Technical Field
This invention relates to packaging of LSI chips and more particularly to the metallurgy of conductor lines and connection pads for solder bonding and wire bonding.
2. Background Art
On the upper surface of a multilayer ceramic chip carrier there is an arrangement of chip sites each with an array of C-4 solder ball pads which are adapted to provide connection to chips by means of bonding of solder balls by heating of Pb/Sn solder balls to a temperature permitting solder bonding of the solder balls to the pads by heating above the bonding temperature of the solder balls carried on the lower surface of the chip to be bonded to the chip carrier. Connected between the solder pad areas and other sites on or in the chip carrier are so-called fan-out lines which extend along the top surface of the chip carrier beneath a layer of insulation. At certain locations on the surface of the chip carrier, it is necessary to make pads available for engineering change wiring to be connected to the fan-out metallurgy. However, the engineering change wiring is connected to the pads by the processes of wire bonding either by ultrasonic vibration or by thermo compression techniques. The metallurgical requirements for solder bonding as contrasted with requirements for wire bonding techniques differ. As a result, there is a metal structure for line connection, a second structure for chip joining and a third structure adapted for engineering change connections or bonds. Heretofore, the top surface metallurgy has been complicated and the materials employed have required many selective metal deposition steps. For example, in one system of chip packaging, the solder pads are composed of a sandwich of Mo--Ni--Au layers and the EC pads and lines are Mo--Ni layers of metal coated with a thick film of Au. Such an array of metallic layers is deposited upon the multiple layers of the fan-out metallurgy. It is necessary under such circumstances to use an entirely different multiple step process for both the solder pads and the EC pads and lines.
U.S. Pat. No. 3,607,379 of Leinkram et al for "Microelectronic Interconnection Substrate" describes an interconnection substrate consisting of an insulative alumina ceramic wafer carrying a first metal layer of chromium, for example, to metallurgically bond a second silicon-eutectic-forming layer such as gold to the substrate. Deposited on the gold layer is a third weldable or tin-lead solderable layer such as a nickel film. Films are photolithographically etched to produce a number of gold and nickel pads suitable for interconnection of microelectronic elements. A lead from a diode is connected by welding to a nickel pad on top of a chromium layer. A gold wire is thermally bonded to an exposed pad on the gold layer. The gold film or pad provides a surface to which resistive elements, semiconductors and other components may be interconnected by means of gold-silicon eutectic formation and by gold wires attached by thermal compression or ultrasonic bonding. The nickel pads provide surfaces to which microcomponents or external leads are tin-lead soldered or welded. Alternatively, the first layer forming an adhesion layer is composed of Cr, Ti, or Al. The second layer is composed of a metal such as Au or Ag. The third or top layer for bonding by thermal compression bonding or ultrasonic bonding is composed of Rh, Ag, Cu, Ni or the like with Ni and Cu preferred as being solderable and weldable and allowing physically stable electrical connections to gold-nickel layers. On the other hand, in accordance with this invention, it is preferred that the solder bonding should be made to metals selected from Au, Cu, or Ni as contrasted to Au or Ag in the case of Leinkram with the metal adapted to ultrasonic or thermal compression solderless bonding in accordance with this invention is selected from Au, Cr, Ti, Al, or Co. The Leinkram patent teaches ultrasonic or thermal compression bonding to Au or Ag. Thus, while some similarity in the metal for solder bonding exists in the case of use of Au on the solder pads, the other metals for solder bonding and thermal compression and ultrasonic bonding metals are quite distinct. Employing the Leinkram design, there is a high possibility of formation of thick intermetallics between the copper conductor and the tin in the solder leading to fracture of the conductor lines when repeated heat cycling is applied.
U.S. Pat. No. 3,843,911 of Horton et al describes a contact pad arrangement with a substrate of copper plated with an intermediate layer selected from Ni, Cr and Mo with the top layer-composed of Au. A gold wire is bonded to the gold layer on the top of the pad by ohmic bonding.
U.S. Pat. No. 4,231,154 of Gadzik "Electronic Package Assembly Method" teaches provision of Cr-Cu-Cr metallurgy for printed circuits on polyimide carrying a C-4 mounted chip bonded by solder bonding. FIG. 4I shows a pad formed of Cu which is formed by stripping away the Cr layer on that portion of the Cr-Cu-Cr stack. The C-4 solder balls are bonded to the Cu layer which has been exposed. There is no suggestion of using a different form of bonding than solder bonding to the upper surface. Pins are bonded to the metallurgy by means of solder bonding also, at different sites.